When a short circuit or current surge occurs on the load of a powered switch, or a switch is powered into a short circuit or heavy load, there occurs an excessive current through the switch. This current can destroy the switch by exceeding the safe-operating area of the switch, and damage the input power supply or cause a voltage transient on the input power supply network which can result in the malfunction of other circuitry due to the undervoltage condition. This problem is often evident when inserting or removing a circuit board under operating conditions (“hot swapping”) into a powered backplane. Current limiting circuits are used to control the initial current and protect against voltage transients when hot swapping.
FIG. 1 shows a circuit diagram for a current limiting circuit used to allow safe hot swapping of a circuit board 10 according to a prior art design. The circuit receives a voltage source Vs 12 and a voltage reference (GND) 14 from a connector 16 when connected to a back plane connector (not shown). A sense resistor Rs 18 is placed in series with a switch 20 between the input voltage source and the board load. The switch connects the voltage source to the board circuits which are indicated by the load capacitor CL 22. A control circuit 24 connects to the gate and controls the switch 20. Inputs to the control circuit are from connections on either side of RS. On the input voltage side of Rs a shunt resistor RSH 26 is used in the connection to the control circuit 24. The control circuit compares the voltage drop on RS and the voltage drop on the shunt resistor 26 to control the input current through the switch 20. The connections to the control circuit are shown with 45 degree connections to indicate Kelvin sense connections, which include a power wire and a sense wire to more accurately read the small voltage differences.
FIG. 2 shows a circuit diagram for a current limiting circuit according to another prior art design. Similar to the previous circuit, the switch connects the voltage source to the board circuits which are indicated by the load capacitor CL 22. A control circuit 24 connects to the gate and controls the switch 20. In this circuit, inputs to the control circuit are from connections on either side of the switch. The control circuit monitors the voltage drop across the switch 20 to control the input current through the switch. In this prior art circuit, the control circuit merely checked for a threshold voltage drop and shut down the circuit for an emergency overload condition.